Log clicks to the above-the-fold file page link

This uses best-effort logging (like most other links do), not the
blocking call that's used by the other file page link, since
analytics folks had severe misgivings about that.

Change-Id: I35204420c834fa4fce3dcf81403cb78b92811caf
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/726
Merge "Log clicks to the above-the-fold file page link"